The present invention relates to a noise suppressing device and, more particularly, to a device which is adapted to suppress the noise generated by a flow of gas passing through a butterfly valve when in an open position thereof.
Conventional butterfly valves comprise a disc plate positioned in a duct for rotation about a transverse axis defined by a shaft in order to control a fluid flow through the duct. Typically, the disc is rotated via a torque applied by an external actuator through the shaft. Butterfly valves have been widely used in the aircraft industry. In an aircraft auxiliary power unit (APU), the load compressor flow is used for different purposes such as main engine start, and cabin air conditioning for aircraft on the ground when the main engine is not in use. This load compressor air is normally diverted and damped into the auxiliary power unit exhaust duct through a ducting with a diverter-valve regulating the flow. The diverter-valve is normally a butterfly valve type for simplicity in design. However, the use of a butterfly valve usually results in undesirable effects such as vibration, wear and noise. For example, high speed gas flow generates aerodynamic throttling noise in frequency regimes between one 1 k Hz-5 k Hz over a wide range of valve openings.
Various solutions have been proposed for reducing the valve noise level, notably the installation of a noise attenuating device, such as a muffler immediately downstream of the butterfly valve. Such butterfly valve and muffler assemblies are well known and widely used in connection with gas turbine engines. However, it has been observed that the muffler deteriorates over time. Moreover, the butterfly valve and muffler assembly, increases the weight and cost of the overall gas turbine engine.
Efforts have also been made to suppress the noise at the source as, for instance, in U.S. Pat. No. 3,238,955 issued to Lassiter, Jr. on Mar. 8, 1966; U.S. Pat. No. 3,960,177 issued to Baumann on Jun. 1, 1976; U.S. Pat. No. 5,465,756 issued to Royalty et al. on Nov. 14, 1995 and U.S. Pat. No. 5,881,995 issued to the Applicant on Mar. 16, 1999. These prior art patents describe various noise attenuating means mounted on the disc plate of the butterfly valve such as pins, ribs, perforated acoustic fences and tabs, which generate turbulence and/or trap vortices to reduce the wake size downstream of the butterfly valve and thereby reduce noise at the source. Although the butterfly valves described in the above mentioned prior art patents are very effective, their ribs, pins, perforated acoustic fences and tabs create an obstruction to the flow, to a certain extent, and increase the torque applied to the disc plate while in operation.
Alternatively, noise attenuation members are provided in the flow passage after the disc plate of the butterfly valve. Pyxc3x6tsixc3xa4 et al. in U.S. Pat. No. 4,691,894 issued on Sep. 8, 1987 describes such a butterfly valve. The noise attenuation members mounted in the flow passage downstream of the disc plate are axially spaced apart and provided with openings extending therethrough. The perforated noise attenuation members radially extend to reduce the cross-sectional area of the flow passage to attenuate the dynamic momentum of the disturbed flow and the noise in the flow passage, and therefore reduce the air pressure loss immediately downstream of the valve. This results in blockage of the flow to a certain extent, which is not desirable, particularly when the butterfly valve is used in an auxiliary power unit for aircraft.
Another acoustic noise elimination technology developed in the aircraft industry is described in U.S. Pat. No. 4,300,656 issued to Burcham on Nov. 17, 1981, for disrupting the continuity of fields of sound pressures forwardly projected from fans or rotors of a type commonly found in a fan or compressor first stage for an air-breathing engine, when operating at tip speeds in the supersonic range. The acoustic noise elimination device includes a sound barrier defined by a plurality of intersecting flat plates having a line of intersection coincident with a longitudinal axis of a tubular cowl, preferably in a cruciform configuration, which serves to disrupt the continuity of rotating fields of noise. Nevertheless, the acoustic noise generated by a fan or compressor is in a form of rotating fields called spinning modes, and is different from the noise generated by the flow wake after a butterfly valve in which there is flow turbulence and no continuous spinning modes exist. This type of sound barrier has never been considered in association with a solution for suppressing butterfly valve noise.
The aircraft industry has been continuously making efforts to improve noise control technology, including suppressing the butterfly valve noise associated with air auxiliary power units used for aircraft. Therefore, there is a need for an improved butterfly valve noise suppressor.
It is one object of the present invention to provide a noise suppression device for a butterfly valve suppressing turbulence in the flow wake behind the butterfly valve plate, which generates the noise sound waves, without creating a substantial obstruction to the flow.
It is another object of the present invention to provide a noise suppression device for a butterfly valve which is at least acoustically equivalent to a prior art muffler used in the butterfly valve but is simpler, lighter durable in operation and inexpensive to manufacture.
It is a further object of the present invention to provide a butterfly valve which is adapted to generate low noise levels.
It is a still further object of the present invention to provide such a butterfly valve which is relatively simple and economical to manufacture.
The present invention generally provides a method for attenuating butterfly valve noise generated by a gas flow passing an edge of a valve plate in a valve housing. The method comprises a step of deploying at least one perforated plate axially extending in a flow path downstream of the valve plate, and thereby dividing the downstream flow path into a plurality of axial segments to permit the gas flow to pass through the axial segments without a substantial obstruction thereto and causes the turbulence in the gas flow wake behind the valve plate, as well as noise sound waves produced by the turbulence, to cross the perforated plate so that the turbulence and the noise sound waves are suppressed.
In a butterfly valve in operation, the valve plate acts as a bluff body. Flow separation occurs along the valve plate edge, forming a substantial wake behind. The turbulence generated in this unsteady wake is the source of noise. The purpose of the present invention is to suppress or minimize the wake as soon as possible by installing at least a perforated and axially extending baffle plate as a flow straightener immediately downstream of the valve. The perforated baffle plate allows the turbulence components in the wake flow to communicate laterally through its surface, hence dissipating the wake unsteadiness, resulting in a better developed mixed flow at the discharge end of the device. The baffle perforation also suppresses the noise sound waves which are produced by the turbulence and propagate along the perforated baffle plate. Nevertheless, the perforated baffle plate does not create a substantial obstruction to the flow because the baffle plate extends in the direction of the flow.
In accordance with another aspect of the present invention, there is a butterfly valve provided, which comprises a valve housing defining a flow path. A closing member is pivotally mounted in the valve housing for rotation about an axis between an open position for allowing the passage of a gas flow through the valve housing and a closed position for blocking the gas flow through the valve housing. The axis extends transversely and centrally across the closing member so that the closing member has a section to pivot in a generally downstream direction when the closing member moves from the closed toward the open position. A means for attenuating noise is mounted to the valve housing immediately downstream of the closing member but without interfering with the section of the closing member pivoting downstream. The means includes a duct defining a downstream flow path of the butterfly valve, and at least one perforated plate extending axially, mounted to the duct, and preferably extending across a centrally longitudinal axis of the duct, to divide a section of the duct into at least two axial segments.
In one embodiment of the present invention, the duct is cylindrical and the means includes a plurality of flat perforated plates mounted in the duct and arranged in a intersecting configuration, preferably conforming to a cruciform, thereby dividing the section of the cylindrical duct into four axial segments each having a quadrant cross-section.
In accordance with a further aspect of the present invention, a noise suppression device is provided for a butterfly valve which has a closing plate pivotally mounted in a valve housing for rotation about an axis to control a gas flow. The noise suppression device comprises a cylindrical duct connected to the valve housing to form a flow path downstream of the butterfly valve, and a plurality of flat and perforated plates for attenuating noise mounted in the cylindrical duct. The plates are arranged in mutually intercepting planes and have a line of intersection coincident with a longitudinal axis of the cylindrical duct.
The features and advantages of the present invention will be better understood with reference to the preferred embodiments described below.